In a direct injection diesel engine wherein fuel is directly injected into a cylinder, it is necessary to promote mixing of the injected fuel and aspirated air in order to enhance the combustibility of the fuel. To satisfy this need, a helical port may for example be used so that the aspirated air sets up a swirl inside the cylinder.
This helical port comprises a straight part, a cylindrical part which opens into a combustion chamber and a winding part bent into a spiral which connects the cylindrical part and straight part.
This helical port is disclosed for example in Tokkai Sho 59-12123 published by the Japanese Patent Office in 1984. According to this prior art, swirl is enhanced and volume efficiency improved by modifying the plan form of the winding part.
However, even when the plan form of the winding part was suitably set, the intensity of the swirl set up in the cylinder is different according to the height of the winding part and it was not necessarily possible to achieve the dual objectives of swirl enhancement and volume efficiency improvement.